The invention relates generally to antenna communications and, more particularly, to multi-beam antenna systems.
The main objective of conventional radio resource management (RRM) algorithms is to maintain the negotiated (or acceptable) QoS. This may be done by always ensuring coverage and optimizing capacity. One important RRM algorithm is Load Control. If the QoS gets below a threshold, the load control may block new users, delay transmission of data or even drop active users.
In a CDMA system, load control is especially important. It maintains system stability and a reasonable link quality level for existing users. If too many users are admitted to a cell, party effects may occur. This will lead to extensive dropping of users in the cell. Also, neighboring cells may also be affected if the inter cell interference is high. Load Control prevents this and maintains system stability. Common for most load control algorithms is that they set a threshold of the usage of a scarce resource. The scarce resource can be an estimate of power (link or total base station transmit), codes, number of users, throughput, SIR based (e.g. the sum of all users"" SIR) and interference level or a combination of these. The estimate can be local (only considering estimates from the associated cell) or can also consider estimates from neighboring sites. The algorithm then admits users as long as the estimate of the scarce resource is not exceeding the threshold.
Generally, conventional adaptive antenna (AA) techniques can be divided into two main categories depending on the type of AA implementation. For a fixed beam (FB) system, there is a set of beams with fixed shape and pointing direction. The second category is a steered or flexible beam implementation where each mobile user has its own beam with adaptive shape and pointing direction, i.e. the beam follows the user as it moves in the cell.
Adaptive antenna techniques can be used to increase the capacity in communication systems, for example a WCDMA system. The spatial dimension used by adaptive antenna concepts enables increased capacity and/or better coverage. FIG. 1 shows an example of how the antenna diagram may look for an adaptive antenna system. In the uplink the C/I is increased and in the downlink the interference is suppressed. Thus, capacity is gained. The inventors have found that in most cases the majority of the downlink interference of a fixed (or steered) multi-beam system for a speech only scenario originates from the main lobe. For example, most of the interference seen by a user connected to the main lobe of beam A in FIG. 1 originates from other users also connected to the main lobe of beam A. However, some sources of interference do not originate from the main lobe of the beam. Examples of such xe2x80x9cexternalxe2x80x9d interference sources include:
1) Other cells;
2) Sidelobe of another beam (see FIG. 1); and
3) An overlap area between the aforementioned main lobe and a main lobe of another beam (see FIG. 1).
For a speech only scenario this external interference typically accounts for only a small part of the total interference for each user. However, for users at the cell border, the interference from other cells can be substantial.
However, third generation systems will include not only speech users, but rather a mix of many different services, such as video, web browsing, file transfers and other such high data rate (HDR) services. The inventors have found that the interference from high data rate users outside the main lobe may be substantial in spite of the attenuation between the sidelobe and main lobe. This is because of either a sidelobe or an overlapping beam area. A high data rate user that interferes substantially with the adjacent main lobe of a beam may cause a congestion problem.
Assume, for example, a steered beam adaptive antenna array wherein each beam direction can handle at maximum an equivalent of M speech users. Assume an admission control that operates on per beam direction. The admission control admits up to the maximum level of M users into the beam direction. This can be done e.g., by checking both the number of equivalent speech users xe2x80x9cconnectedxe2x80x9d to the beam and/or the interference load on the beam. Consider the example in FIG. 2 with three beams, A, B and C, whose main lobes are directed outwardly from the antenna array in respective radial directions. A high data rate user is admitted access to beam B. Further, assume that the total load in beam B equals exactly M equivalent speech users. However, when the high data rate user connected to beam B moves into the side lobe of beam A it will be affected by the transmit power from the speech users from beam A. Thus, the HDR user will experience increased interference. There is an xe2x80x9cinterference leakagexe2x80x9d between the beams.
The high data rate user must increase its required transmit power due to the new increased interference from the speech users in beam A. The increased transmit power from the high data rate user will in turn affect all other users xe2x80x9cconnectedxe2x80x9d or adjacent to beam B. In particular, the increased transmit power from the HDR will increase the interference for all users xe2x80x9cconnected toxe2x80x9d or adjacent to beam B. Assume now that the increased interference corresponds to an equivalent of N1 speech users, so beam B now has M+N1 equivalent speech users. This will very likely result in a very severe and non-acceptable quality problem and a sharp increase of required power from the users, which will lower the total system capacity.
A similar situation may occur due to the overlapping area of main lobes. Once again consider the example in FIG. 2 with only three beams. In beam B a high data rate user is admitted access. When the high data rate user connected to beam B moves into the overlapping area of the main lobes of beams A and B, it will cause, e.g., an interference equivalent of N2 equivalent speech users in beam A. Thus, there is once again an xe2x80x9cinterference leakagexe2x80x9d between the beams.
Conventional congestion control may eventually resolve the above problems by dropping some of the users, but the capacity and quality in the system will be lowered.
Assuming the maximum number of equivalent speech users in each beam is M, another solution may be to set a new admission control level of Q speech equivalents, lower than M, e.g. Q+N=M, where N is the possible external interference (in speech equivalent). This reduces system capacity.
It is therefore desirable to avoid the aforementioned xe2x80x9cinterference leakagexe2x80x9d between beams (e.g. due to the sidelobes and the overlapping areas of main lobes) without degrading the capacity and quality of the system.
The invention exploits the situation wherein the load/interference from an angular perspective is not equal within a cell, and spreads the interference more equally within the cell. This is achieved by judicious use of at least one of nulling, beam steering and beam selection. The load/interference spreading decreases the load in highly loaded areas/directions and increases the load in less heavily loaded areas/directions. The nulling, beam steering and beam selection operations are applicable to coherent adaptive antenna systems, and the beam selection technique is also applicable to fixed beam systems.